In mining and stone industries, it is in many cases important to fractionate crushed stone and gravel into fractions of stones with different sizes. Normally, a deviation from the size is permitted according to industry standards, e.g., 10 percent oversized particles and 15 percent undersized particles.
In most cases, fractionating is done by supplying an unfractionated stream of crushed stone or gravel to a vibrating screen provided with a screening deck including screening holes for allowing stones smaller than the screening holes to pass through the holes. The vibration pattern and the inclination of the vibrating screen are arranged so that the crushed stones continuously flow in one direction on the screen, ultimately leaving it on one side or falling through the holes in the screening deck.
To improve the screening of the unfractionated material and to get a thicker material layer on the screening deck some screening devices are provided with raised portions, see e.g., SE-C2-524 179 and EP-B1-0 680 386. In SE-C2-524 179, stiffening longitudinal beams are arranged on top of the screening deck to reduce the wear on the screening holes and to make undersized particles to pass faster through the screening deck. In EP-B1-0 680 386, a curved surface in the form of longitudinal ridges has been arranged on top of the screening deck, where the curved surface is a fine screening screen. The curved surface aims to provide a larger screening surface and to prevent material from being screened to migrate toward the lower sides of the screen.
Another application of a non-flat screening deck is disclosed in U.S. Pat. No. 6,607,080 and U.S. Pat. No. 6,629,610, where systems for separating particles from a mix of particles and fluids by a screen with shale shakers used in the drilling industry are shown. The screen is designed with a flat screening deck and with ramps so that a portion of the fluids will pass over the ramp, whereas the screen will separate the particles from the remaining portion containing both particles and fluid.
The problem with the above prior art solutions and screening devices having only a flat screening deck is that none of them reduces the traveling speed of the material to be stratified as it passes over the screening deck. Furthermore, they do not include elements for stirring or mixing the material to improve the screening.
Faced with the above prior art screening systems and the disadvantages and problems therewith, the object with the present invention is to provide a solution to improve screening efficiency in a simple way with a screening assembly for a vibrating screen for screening of unfractionated material, such as crushed stone and gravel.